Magnetic recording disk drives incorporate stacked, commonly-rotated rigid magnetic recording disks that are used for storage of user data. The data is recorded in radially-spaced data tracks on the surfaces of the disks. Recording heads are moved by an actuator in a generally radial path toward and away from the center of rotation of the disks to read and write data on the disks. Typically, a single recording head, which may be an inductive read/write head or an inductive write head in combination with a magnetoresistive read head, is associated with a corresponding magnetic recording surface of each disk.
It is necessary to know the precise radial and circumferential location of the recording heads relative to their associated disk surfaces. Radial position information is recorded on the disk as servo information and is used to locate the heads to the desired data tracks and maintain the heads over the desired data tracks during read and write operations. Circumferential position information is used to identify the start of different data fields located around the tracks. In conventional fixed-block architecture disk drives, the data is located in fixed-length angular data sectors and the servo information is located in angularly-spaced servo sectors that are interspersed among the data sectors. This type of disk drive is referred to as a sector servo or embedded servo disk drive.
In embedded servo disk drives, each of the servo sectors contains a servo timing mark (STM) that is read by the head to initiate the subsequent detection of servo information in the form of position error signal (PES) bursts. The PES bursts are decoded by the servo electronics to calculate the radial position of the head and provide feedback to the actuator to reposition the head to maintain it along the centerline of the desired track. In a specific type of fixed-block architecture embedded servo disk drive that incorporates a "No-ID".TM. brand of headerless architecture where there are no identification (ID) fields to locate the data sectors, the STMs are also used to assist in locating the specific data sectors where user data is to be read or written. This type of disk drive is described in IBM's U.S. Pat. No. 5,500,848.
Accurate detection of STMs is crucial to proper disk drive operation since it is necessary to correctly recognize subsequent servo information (PES bursts) and track ID information that are located in the servo sectors after the STMs. If a servo sector is not recognized because of errors in the STM, the servo electronics will generate servo timing information based on previous STMs and servo tracking and timing accuracy will be diminished. Also, if the servo electronics incorrectly detects an STM at the wrong location because of an error condition, incorrect data will be sent to the track ID decoder and PES decoder, resulting in missed or incorrectly interpreted track ID and head position. Correct identification of the STM located at the index (the beginning of the track) is also important in disk drives with headerless architectures because the location of the appropriate data sectors for reading and writing data begins with identification of the index STM.
Prior art disk drives attempted to improve the reliability of reading servo timing marks by the use of redundant STM bit patterns adjacent to one another and the use of specific types of a single bit pattern for all STMs that can tolerate up to a fixed number of bit errors. These methods are inefficient and do not allow the use of multiple STM patterns capable of being accurately decoded by the servo electronics. Moreover, none of the prior art disk drives allow any modification in the STM correction and detection power.
What is needed is a disk drive that can accurately detect multiple types of STM patterns so that STMs can be used to identify different parameters, such as track ID, track index and head number, even when any one or more of these multiple STM patterns contains bit errors, and that can be modified or adapted to alter the error correction and/or detection power.